Missing passport photo for H4 EAD might result in RFE
Yes, the missing photo might result in H4 EAD RFE.
H4 EAD and H4 extension are separate applications
Yes, both applications are separate applications.
H4 EAD RFE will not affect your H4 extension application.
In-fact, your H4 EAD application will not even be picked up for work unless H4 Extension has reached a result.
Premium processing for H4 EAD RFE
Since you filed your H1B + H4 + H4 EAD in same bundle with premium processing, the chances are high that they all will be processed in premium.
But, if you receive H4 EAD RFE, the chances of it being treated in Premium processing after RFE are low.